1. Field of the Invention
The invention relates to a process for preparing trichlorosilane by means of thermal hydrogenation in the supercritical pressure range.
2. Description of the Related Art
The reaction of trichlorosilane with hydrogen to produce polycrystalline silicon results in formation of large amounts of tetrachlorosilane. The tetrachlorosilane can be converted back into trichlorosilane and hydrogen chloride by tetrachlorosilane converting, namely a catalytic or thermal dehydrohalogenation reaction of tetrachlorosilane with hydrogen. Two process variants are known for converting tetrachlorosilane into trichlorosilane. Low-temperature converting is carried out in the presence of silicon and a catalyst at temperatures in the range from 400° C. to 700° C. The U.S. Pat. Nos. 2,595,620, 2,657,114 (Union Carbide and Carbon Corporation/Wagner 1952) and U.S. Pat. No. 2,943,918 (Compagnie de Produits Chimiques et electrometallurgiques/Pauls 1956) report a partial hydrogenation of tetrachlorosilane in the presence of catalysts (e.g. metallic chlorides).
Since the presence of catalysts, e.g. copper, can lead to contamination of the trichlorosilane and the polycrystalline silicon produced therefrom, a second process, for example the high-temperature process, has been developed. In this process, the starting materials tetrachlorosilane and hydrogen are reacted without catalyst at higher temperatures than in the low-temperature process to form trichlorosilane. Tetrachlorosilane converting is an endothermic process in which the promotion of the products is equilibrium limited. To obtain a significant yield of trichlorosilane at all, high temperatures (>900° C.) have to prevail in the reactor. Thus, U.S. Pat. No. 3,933,985 (Motorola INC/Rodgers 1976) describes the reaction of tetrachlorosilane with hydrogen to form trichlorosilane at temperatures in the range from 900° C. to 1200° C. and a molar ratio of H2:SiCl4 of from 1:1 to 3:1. Trichlorosilane yields of 12-13% are achieved in this reaction.
U.S. Pat. No. 4,217,334 (Degussa/Weigert 1980) describes an optimized process for converting tetrachlorosilane into trichlorosilane by hydrogenation of tetrachlorosilane by means of hydrogen in a temperature range from 900° C. to 1200° C. As a result of a high molar ratio of H2:SiCl4 (up to 50:1) and a liquid quench of the hot product gas to below 300° C. (liquid: product or inert liquid, cooling times: 50 ms), significantly higher trichlorosilane yields (up to about 35% at an H2:SiCl4 ratio of 5:1) are achieved. Disadvantages of this process are the significantly higher proportion of hydrogen in the reaction gas and the quench by means of a liquid, since both these greatly increase the energy consumption and thus the costs of the process. The quench is necessary in order to “freeze” the reaction equilibrium which is on the SiHCl3 and HCl side, and is effected by immediate quenching with SiCl4 from 1100° C. to 300° C., which is energetically unsatisfactory and therefore expensive.
The abstract of JP60081010 (Denki Kagaku Kogyo K.K./1985) describes a quenching process at lower H2:SiCl4 ratios to increase the trichlorosilane content in the product gas. The temperatures in the reactor are from 1200° C. to 1400° C. The reaction mixture is cooled to below 600° C. within one second. In this quenching process, too, most of the energy of the reaction gas is lost, which has a serious adverse effect on the economics of the process.
DE 3024319 describes a continuous process in which a mixture of tetrachlorosilane and hydrogen reacts at 900-1300° C. in a high-temperature reactor and in which the hydrogen chloride formed is, after cooling in an after-reactor, reacted over a silicon catalyst at from 280 to 350° C. to form further trichlorosilane. The unreacted tetrachlorosilane and the unreacted hydrogen are recirculated to the high-temperature reactor. This process is preferably carried out at from 1 to 6 bar. To increase the energy efficiency of the process, a heat exchanger unit is integrated into the high-temperature reactor in DE 3024319.
Owing to the increasing economic importance of the production of polycrystalline silicon, e.g. for photovoltaics, and continually increasing energy prices, increased efforts have been made in recent years to make the primary energy usage in silane converting based on the trichlorosilane yield more efficient.